Oxidation inhibitor for diesel, and diesel fuel composition

ABSTRACT

The present invention provides an oxidation inhibitor for diesel that improves the oxidative stability thereof and a diesel fuel composition having an excellent oxidative stability. A diesel fuel composition excellent in oxidative stability and low-temperature fluidity is obtained by adding an oxidation inhibitor that contains a fatty acid methyl ester derived from palm, rapeseed or soybean oil and in which the contents of polyunsaturated and saturated fatty acid methyl esters are each specified.

TECHNICAL FIELD

The present invention relates to an oxidation inhibitor for diesel, and a diesel fuel composition excellent in oxidative stability and low-temperature fluidity.

BACKGROUND ART

Diesel oil used as diesel car fuel and the like is known to show, for example, discoloration, formation of polymer deposit (sludge) and increase in viscosity when oxidized. It is also known that peroxide generated by oxidation deteriorates members such as rubber in a fuel system. Therefore, there is a need to improve the oxidative stability of diesel. In order to improve the oxidative stability of diesel, a method in which amine and phenolic oxidation inhibitors and the like are added to diesel has been used (Patent Literature 1).

Recently, reduction of sulfur compounds and aromatic hydrocarbons (particularly polycyclic aromatic hydrocarbons) in fuels is required from the viewpoints of, for example, preventing poisoning of exhaust gas purifying catalysts, reducing toxic substances in exhaust gas, and regulating fuel consumption. However, since these sulfur compounds or aromatic hydrocarbons themselves have an oxidation inhibiting effect, reduction of these compounds can cause lowering of the oxidative stability. Furthermore, strengthened regulations on exhaust gas from diesel vehicle have further promoted higher-pressure fuel injection with common rail, which increase the thermal load on diesel oil, and thus diesel oil is required to have increased oxidative stability more than ever. In order to meet the required level, it is necessary to add large amounts of oxidation inhibitors, which will increase manufacturing cost. Furthermore, addition of larger amounts of oxidation inhibitors may cause a problem that they easily precipitate when temperature drops (Patent Literature 2).

Thus, a diesel fuel composition that maintains oxidative stability of diesel fuel without using an oxidation inhibitor has been proposed (Patent Literature 3). Specifically, fluorenes and naphthenobenzenes that are focused as substances having poor oxidative stability are mixed with naphthalenes that are substances having good oxidative stability, and their contents are adjusted to ensure oxidative stability. However, since the aromatic components have low cetane numbers, many of them generally have poor ignitability and cause deterioration of flammability. This raises the concern that the formation of particulate matter can be increased.

PRIOR ART REFERENCES Patent Literature

Patent Literature 1: Japanese Laid-open Patent Application (Kokai) No. 2004-225000

Patent Literature 2: Japanese Patent No. 5427361

Patent Literature 3: Japanese Laid-open Patent Application (Kokai) No. 2011-184672

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Thus, the problem to be solved by the invention is to provide an oxidation inhibitor for diesel that improves the oxidative stability thereof and a diesel fuel composition having an excellent oxidative stability.

Means for Solving the Problems

As a result of intensive studies for solving the above problem, the present inventors found that a diesel fuel composition excellent in oxidative stability and low-temperature fluidity is obtained by adding an oxidation inhibitor that contains a fatty acid methyl ester derived from palm oil, rapeseed oil or soybean oil and in which the contents of polyunsaturated and saturated fatty acid methyl esters are each specified, and thereby completed the present invention.

Thus, according to the present invention, the following inventions are provided.

As oxidation inhibitors for diesel, provided are:

[1] an oxidation inhibitor for diesel comprising a palm oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.5-7.0 mass % of polyunsaturated fatty acid methyl ester and 50-84 mass % of saturated fatty acid methyl ester;

[2] an oxidation inhibitor for diesel comprising a rapeseed oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.5-24.5 mass % of polyunsaturated fatty acid methyl ester and 7-50 mass % of saturated fatty acid methyl ester; and

[3] an oxidation inhibitor for diesel comprising a soybean oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.8-32 mass % of polyunsaturated fatty acid methyl ester and 15-56 mass % of saturated fatty acid methyl ester.

As a diesel fuel composition, provided is

[4] a diesel fuel composition, which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the oxidation inhibitor for diesel described in [1], the oxidation inhibitor for diesel described in [2], and the oxidation inhibitor for diesel described in [3], in an amount of 1.0 mass % or more and 70 mass % or less.

Also, as a diesel fuel composition having high oxidative stability and very high low-temperature fluidity, provided is

[5] the diesel fuel composition described in [4], which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the oxidation inhibitor for diesel described in [1], the oxidation inhibitor for diesel described in [2], and the oxidation inhibitor for diesel described in [3], in an amount of 1.0 mass % or more and 20 mass % or less.

Also, as a diesel fuel composition having higher oxidative stability and high low-temperature fluidity, provided is [6] the diesel fuel composition described in [4], which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the oxidation inhibitor for diesel described in [1], the oxidation inhibitor for diesel described in [2], and the oxidation inhibitor for diesel described in [3], in an amount of more than 20 mass % and 50 mass % or less.

Also, as a diesel fuel composition having very high oxidative stability, provided is

[7] the diesel fuel composition described in [4], which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the oxidation inhibitor for diesel described in [1], the oxidation inhibitor for diesel described in [2], and the oxidation inhibitor for diesel described in [3], in an amount of more than 50 mass % and 70 mass % or less.

Characteristics of the above-mentioned diesel fuel composition are as follows:

[8] the diesel fuel composition described in [4], which satisfies all the following conditions (a), (b) and (c):

(a) the pour point is 9° C. or less;

(b) the oxidative stability by PetroOxy method is 65 minutes or more;

(c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.

Characteristics of the above-mentioned diesel fuel composition having high oxidative stability and very high low-temperature fluidity are as follows:

[9] the diesel fuel composition described in [5], which satisfies all the following conditions (a-1), (b) and (c):

(a-1) the pour point is −8° C. or less;

(b) the oxidative stability by PetroOxy method is 65 minutes or more;

(c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.

Characteristics of the above-mentioned diesel fuel composition having higher oxidative stability and high low-temperature fluidity are as follows: [10] the diesel fuel composition described in [6], which satisfies all the following conditions (a-2), (b) and (c):

(a-2) the pour point is 4° C. or less;

(b) the oxidative stability by PetroOxy method is 65 minutes or more;

(c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.

Characteristics of the above-mentioned diesel fuel composition having very high oxidative stability are as follows:

[11] The diesel fuel composition described in [7], which satisfies all the following conditions (a), (b) and (c):

(a) the pour point is 9° C. or less;

(b) the oxidative stability by PetroOxy method is 65 minutes or more;

(c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.

Effect of the Invention

According to the present invention, an oxidation inhibitor for diesel that improves the oxidative stability thereof as well as a diesel fuel composition excellent in oxidative stability and low-temperature fluidity can be provided.

EMBODIMENTS FOR CARRYING OUT THE INVENTION Oxidation Inhibitor for Diesel

An oxidation inhibitor for diesel according to an aspect of the present invention is an oxidation inhibitor for diesel comprising a palm oil-derived fatty acid methyl ester (hereinafter sometimes abbreviated as “oxidation inhibitor 1”), an oxidation inhibitor for diesel comprising a rapeseed oil-derived fatty acid methyl ester (hereinafter sometimes abbreviated as “oxidation inhibitor 2”), or an oxidation inhibitor for diesel comprising a soybean oil-derived fatty acid methyl ester (hereinafter sometimes abbreviated as “oxidation inhibitor 3”), and satisfies the following conditions:

(1) a condition for the oxidation inhibitor 1 (comprising a palm oil-derived fatty acid methyl ester) where

the content of polyunsaturated fatty acid methyl ester is 0.5-7.0 mass % and the content of saturated fatty acid methyl ester is 50-84 mass %;

(2) a condition for the oxidation inhibitor 2 (comprising a rapeseed oil-derived fatty acid methyl ester) where

the content of polyunsaturated fatty acid methyl ester is 0.5-24.5 mass % and the content of saturated fatty acid methyl ester is 7-50 mass %;

(3) a condition for the oxidation inhibitor 3 (comprising a soybean oil-derived fatty acid methyl ester) where

the content of polyunsaturated fatty acid methyl ester is 0.8-32 mass % and the content of saturated fatty acid methyl ester is 15-56 mass %.

The present inventors, as a result of intensive studies for seeking oxidation inhibitors that improve the oxidative stability of diesel, have found that a diesel fuel composition excellent in oxidative stability and low-temperature fluidity is obtained by adding an oxidation inhibitor that comprises a fatty acid methyl ester derived from palm oil, rapeseed oil or soybean oil and in which the contents of polyunsaturated fatty acid methyl esters and saturated fatty acid methyl esters are each in the specific range.

An oxidation inhibitor containing polyunsaturated and saturated fatty acid methyl ester tends to have a higher antioxidant effect and a lower low-temperature fluidity when it contains a lower content of polyunsaturated fatty acid methyl ester and a higher content of saturated fatty acid methyl ester. Since the variation in the fatty acid compositions depends on the type of oils used as the raw materials, the content of each of polyunsaturated and saturated fatty acid methyl esters depends on the type of oils as the raw materials.

Thus, the present inventors have found that when a fatty acid methyl ester derived from palm oil, rapeseed oil or soybean oil is used in an oxidation inhibitor for diesel, a diesel fuel composition excellent in oxidative stability and low-temperature fluidity can be provided by adjusting the contents of the polyunsaturated and saturated fatty acid methyl esters depending on the oil type.

The phrase “comprising a fatty acid methyl ester derived from . . . ” means that the oxidation inhibitor may also contain, for example, polyunsaturated fatty acid methyl ester and saturated fatty acid methyl ester synthesized from petroleum-derived raw material or the like, polyunsaturated fatty acid methyl ester and saturated fatty acid methyl ester obtained by using other vegetable oils, or impurities as long as the oxidation inhibitor contains a fatty acid methyl ester obtained by using said type of oil. The method for obtaining fatty acid methyl esters from vegetable oil is not particularly limited, and any known methods can be used as appropriate, including specifically the use of transesterification reaction of fat with methanol.

“Polyunsaturated fatty acid methyl ester” means unsaturated fatty acid methyl esters having two or more carbon-carbon double bonds in the molecules.

The oxidation inhibitor 1, oxidation inhibitor 2 and oxidation inhibitor 3 will be described in detail below.

The oxidation inhibitor 1 is an oxidation inhibitor comprising a palm oil-derived fatty acid methyl ester, and the content of the palm oil-derived fatty acid methyl ester in the oxidation inhibitor 1 is usually 96.5 mass % or more, preferably 97.0 mass % or more, more preferably 98.0 mass % or more.

In the oxidation inhibitor 1, the content of polyunsaturated fatty acid methyl ester is 0.5-7.0 mass %, preferably 0.6 mass % or more, more preferably 0.7 mass % or more, and preferably 5.0 mass % or less, more preferably 3.0 mass % or less.

In the oxidation inhibitor 1, the content of saturated fatty acid methyl ester is 50-84 mass %, preferably 52 mass % or more, more preferably 54 mass % or more, and preferably 77% mass % or less, more preferably 70 mass % or less.

The oxidation inhibitor 1 may contain an unsaturated fatty acid methyl ester (monoenoic acid methyl ester) having one carbon-carbon double bond in the molecule, and the content of the monoenoic acid methyl ester is usually 14 mass % or more, preferably 18 mass % or more, more preferably 20 mass % or more, and usually 46 mass % or less, preferably 43 mass % or less, more preferably 40 mass % or less.

The oxidation inhibitor 1 may contain compounds other than fatty acid methyl ester, and the content of the compounds other than fatty acid methyl ester is usually 3.5 mass % or less, preferably 3.0 mass % or less, more preferably 2.0 mass % or less.

Within the above range, it is easy to ensure good oxidative stability and low-temperature fluidity.

The oxidation inhibitor 2 is an oxidation inhibitor comprising a rapeseed oil-derived fatty acid methyl ester, and the content of the rapeseed oil-derived fatty acid methyl ester in the oxidation inhibitor 2 is usually 96.5 mass % or more, preferably 97.0 mass % or more, more preferably 98.0 mass % or more.

In the oxidation inhibitor 2, the content of polyunsaturated fatty acid methyl ester is 0.5-24.5 mass %, preferably 1.0 mass % or more, more preferably 1.5 mass % or more, and preferably 10 mass % or less, more preferably 4.0 mass % or less.

In the oxidation inhibitor 2, the content of saturated fatty acid methyl ester is 7-50 mass %, preferably 10 mass % or more, more preferably 15 mass % or more, and preferably 40 mass % or less, more preferably 30 mass % or less.

The oxidation inhibitor 2 may contain an unsaturated fatty acid methyl ester (monoenoic acid methyl ester) having one carbon-carbon double bond in the molecule, and the content of the monoenoic acid methyl ester is usually 45 mass % or more, preferably 50 mass % or more, more preferably 60 mass % or more, and usually 80 mass % or less, preferably 75 mass % or less, more preferably 70 mass % or less.

The oxidation inhibitor 2 may contain compounds other than fatty acid methyl ester, and the content of the compounds other than fatty acid methyl ester is usually 3.5 mass % or less, preferably 3.0 mass % or less, more preferably 2.0 mass % or less.

Within the above range, it is easy to ensure good oxidative stability and low-temperature fluidity.

The oxidation inhibitor 3 is an oxidation inhibitor comprising a soybean oil-derived fatty acid methyl ester, and the content of the soybean oil-derived fatty acid methyl ester in the oxidation inhibitor 3 is usually 96.5 mass % or more, preferably 97.0 mass % or more, more preferably 98.0 mass % or more.

In the oxidation inhibitor 3, the content of polyunsaturated fatty acid methyl ester is 0.8-32 mass %, preferably 1.0 mass % or more, more preferably 1.2 mass % or more, and preferably 10 mass % or less, more preferably 4.0 mass % or less.

In the oxidation inhibitor 3, the content of saturated fatty acid methyl ester is 15-56 mass %, preferably 17 mass % or more, more preferably 20 mass % or more, and preferably 40 mass % or less, more preferably 35 mass % or less.

The oxidation inhibitor 3 may contain an unsaturated fatty acid methyl ester (monoenoic acid methyl ester) having one carbon-carbon double bond in the molecule, and the content of the monoenoic acid methyl ester is usually 25 mass % or more, preferably 40 mass % or more, more preferably 60 mass % or more, and usually 75 mass % or less, preferably 70 mass % or less, more preferably 65 mass % or less.

The oxidation inhibitor 3 may contain compounds other than fatty acid methyl ester, and the content of the compounds other than fatty acid methyl ester is usually 3.5 mass % or less, preferably 3.0 mass % or less, more preferably 2.0 mass % or less.

Within the above range, it is easy to ensure good oxidative stability and low-temperature fluidity.

The method for adjusting the contents of polyunsaturated and saturated fatty acid methyl esters in the oxidation inhibitors 1-3 is not particularly limited, and any known method can be used as appropriate, including any one of (1) to (3) below:

(1) a method in which a polyunsaturated fatty acid methyl ester and/or an unsaturated fatty acid methyl ester (monoenoic acid methyl ester) is/are hydrogenated;

(2) a method in which a palm oil, rapeseed oil or soybean oil containing specific amounts of polyunsaturated and saturated fatty acids is selected to obtain fatty acid methyl esters; and

(3) a method in which a mixture of saturated fatty acid methyl esters, unsaturated saturated fatty acid methyl esters (monoenoic acid methyl esters), highly saturated fatty acid methyl esters, and the like are added.

When a saturated fatty acid methyl ester is added, the carbon number of the saturated fatty acid methyl ester is usually 8 or more, preferably 10 or more, more preferably 12 or more, and usually 22 or less, preferably 20 or less, more preferably 18 or less. Specific examples of the saturated fatty acid methyl esters include methyl esters of capric acid, lauric acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, arachidic acid and behenic acid.

When a monoenoic acid methyl ester is added, the carbon number of the monoenoic acid methyl ester is usually 8 or more, preferably 10 or more, more preferably 12 or more, and usually 22 or less, preferably 20 or less, more preferably 18 or less. Specific examples of the monoenoic acid methyl ester include methyl esters of palmitoleic acid, oleic acid, vaccenic acid, eicosenoic acid and erucic acid.

When a fatty acid methyl ester obtained from an oil type other than palm oil, rapeseed oil and soybean oil is added, examples of the oil type include vegetable oils such as Jatropha curcas oil, safflower oil, sunflower oil, olive oil, cottonseed oil, tung oil, crude palm oil and coconut oil, and animal oils such as fish oil.

Diesel Fuel Composition

As described above, a diesel fuel composition excellent in oxidative stability and low-temperature fluidity can be obtained by adding the oxidation inhibitor 1, 2 or 3. A diesel fuel composition comprising at least one oxidation inhibitor for diesel selected from the group consisting of the oxidation inhibitors 1, 2 and 3 in an amount of 1.0 mass % or more and 70 mass % or less is also one aspect of the present invention (hereinafter sometimes abbreviated as “diesel fuel composition of the present invention”).

The diesel fuel composition of the present invention comprises at least one oxidation inhibitor for diesel selected from the group consisting of the oxidation inhibitors 1, 2 and 3 in an amount of 1.0 mass % or more and 70 mass % or less. Further, although the lower limit of the oxidation inhibitor for diesel is 1.0 mass % or more, it is 5.0 mass % or more in order to exhibit a higher antioxidant effect, and it is 10.0 mass % or more in order to exhibit an even higher antioxidant effect. Thus, the oxidation inhibitor can inhibit sludge formation after forced-oxidation when the total content is 1.0 mass % or more, have a higher oxidative stability when the total content is 5.0 mass % or more, and have an even higher oxidative stability when the total content is 10.0 mass % or more. The total content may also be more than 20 mass %, or more than 50 mass %.

The diesel fuel composition of the present invention may be any composition comprising a specific amount of the above-mentioned oxidation inhibitor for diesel. In Japan, JIS K2204 defines No. 2 diesel as one having a pour point of −7.5° C. or less and Special No. 1 diesel as one having a pour point of 5° C. or less. On the other hand, in warm Thailand and Indonesia where large quantities of fatty acid methyl esters are produced, the pour points are defined respectively as 10° C. or less (Thailand: High Speed) and as 18.3° C. or less (Indonesia: automotive), which are higher than Japan. If the pour point is −8° C. or less, it satisfies −7.5° C. or less of pour point, and if the pour point is 4° C. or less, it satisfies 5° C. or less of pour point for Special No. 1 diesel. Further, if the pour point is 9° C. or less, it satisfies the pour point standards in both Thailand (High Speed) and Indonesia (automotive). Thus, 70 mass % or less of the oxidation inhibitor for diesel added can achieve 9° C. or less of pour point which conforms the above-mentioned standards in warm Thailand and Indonesia, 50 mass % or less of the oxidation inhibitor for diesel added can achieve 4° C. or less of pour point which conform the standard for Special No. 1 diesel, and 20 mass % or less of the oxidation inhibitor for diesel added can achieve −8° C. or less of pour point which conform the standard for No. 2 diesel.

As a mandatory standard of mixed diesel in which more than 0.1 mass % and 5 mass % or less of fatty acid methyl ester is mixed with diesel, the enforcement regulation on the quality assurance of volatile oil and the like was revised on Dec. 20, 2013, in which PetroOxy method is used as an index of oxidative stability. This method also defines the oxidative stability as 65 minutes or more.

Thus, the diesel fuel composition of the present invention preferably has an oxidative stability of 65 minutes or more by PetroOxy method.

PetroOxy method can evaluate oxidative stability, but it does not give any finding on sludge formation. Therefore, after the test method for oxidative stability evaluation prescribed in Ministry of Economy, Trade and Industry Notification No. 81, 2007, which is the former forced-oxidation method capable of simultaneously evaluating sludge generation, the diesel fuel composition of the present invention preferably does not generate any sludge.

A mixed diesel in which the increase in oxidation is 0.12 mg KOH/g or less as measured by this method is supposed to satisfy the requirement of oxidative stability of 65 minutes or more as measured by the PetroOxy method after the revision. In this test method, the increase in acid value after forced oxidation treatment under pure oxygen flow is specified to be 0.12 mg KOH/g or less as described above. On the other hand, although the above specifications are not provided in the case where fatty acid methyl ester is mixed into diesel in an amount of 5 mass % or more, the increase in acid value preferably is 0.12 mg KOH/g or less as in the case mixed in amount of 0.1 to 5 mass %.

The oxidation inhibitor for diesel and the diesel fuel composition in the present invention may also contain, within a range not to impair the objects of the present invention, fuel additives such as flow improver, lubricity improver, pour point depressant, cetane number improver, oxidation inhibitor, metal deactivator, detergent, corrosion inhibitor, de-icer, microbicide, combustion improver, antistatic agent, and colorant.

EXAMPLES

The present invention will be described below based on Examples and Comparative Examples, but the present invention is not limited thereto. First, measurement methods of pour point and oxidative stability, and composition of fatty acid methyl esters in stock oil used in the examples are described.

Measurement of Pour Point

Pour points were measured using an automatic pour/cloud point tester (type MPC-102A, manufactured by Tanaka Scientific Limited) in compliance with US standard ASTM D 6749.

Measurement of Oxidative Stability

Oxidative stabilities were measured by the PetroOxy method. In this method, 5 ml of a sample is placed in the sample chamber, oxygen is injected into the chamber to 700 kPa±5 kPa, and then the temperature is raised to 140.0° C.±0.5° C. and held.

Oxygen is consumed due to oxidative deterioration of the sample with the lapse of time and the pressure inside the sample chamber drops. The oxidative stability means the time elapsed from the start of the temperature rise to the pressure drop point (the point at which the pressure inside the sample chamber dropped by 10% from the maximum pressure) during continuous pressure change measurement.

Sludge formation test was carried out by former forced-oxidation method. In the method, 20 g of a sample was placed in a reaction vessel, heated to 115° C. while feeding pure oxygen into the reaction vessel at 100 ml/min and allowed to oxidize for 16 hours. Acid values before and after the oxidation were measured and the difference therebetween was calculated to obtain the increase in acid value. The acid values were measured using an automatic titrator (type Titrand, manufactured by Metrome). When the sample was held at room temperature after forced oxidation, presence of sludge formation and change in oil color were also investigated.

Stock Oil

Fatty acid methyl esters (FAME) used as oxidation inhibitors in the present invention were palm oil FAME, rapeseed oil FAME and soybean oil FAME. Palm oil FAME used was obtained from Thailand. The other FAME was prepared by ourselves from stock oil by the alkali catalyst method. Into a glass autoclave 150 g of these FAME were added together with 1.2 g of a commercially available hydrogenation catalyst and hydrogenated at a hydrogen pressure of 0.5 MPa and 80° C., and then samples were taken at the specific times, thereby changing the FAME composition. The FAME composition and pour point of each sample are shown in Tables 1 to 3. Methyl stearate (purity: 99% or more) and methyl oleate (purity: 99% or more) as reagents were added to palm oil FAME-1 to adjust the amount of polyunsaturated FAME. The FAME composition and pour point obtained by the adjustment are shown in Table 4.

TABLE 1 FAME palm oil FAME composition (%) FAME-1 FAME-2 FAME-3 FAME-4 FAME-5 FAME-6 FAME-7 FAME-8 saturated FAME 50.0 50.0 50.7 54.2 59.4 72.0 77.6 84.0 monoenoic acid ME 40.5 42.2 45.4 42.7 39.0 26.7 21.0 14.7 polyunsaturated FAME 9.2 7.0 3.1 2.1 1.0 0.7 0.5 0.5 pour point (° C.) 13 13 13 13 14 15 17 19

TABLE 2 FAME rapeseed oil FAME composition (%) FAME-9 FAME-10 FAME-11 FAME-12 FAME-13 EAME-14 FAME-15 saturated FAME 7.0 7.0 8.1 13.6 23.3 36.5 49.8 monoenoic acid ME 63.3 67.1 78.8 81.2 73.6 61.5 48.7 polyunsaturated FAME 28.6 24.5 11.9 3.4 1.5 0.9 0.5 pour point (° C.) −13 −12 −6 5 13 20 25

TABLE 3 FAME soybean oil FAME composition (%) FAME-16 FAME-17 FAME-18 FAME-19 FAME-20 FAME-21 saturated FAME 15.1 15.9 17.8 27.0 39.3 55.9 monoenoic acid ME 24.7 51.9 76.4 70.8 59.0 42.5 polyunsaturated FAME 59.2 34.7 4.4 1.4 1.2 0.8 pour point (° C.) 0 0 3 12 18 24

TABLE 4 palm oil FAME + reagent FAME-22 FAME-23 FAME-24 FAME composition (%) saturated FAME 60.0 60.0 60.0 monoenoic acid 45.0 47.0 49.0 ME polyunsaturated 5.0 3.0 1.0 FAME pour point (° C.) 16 15 14

Examples 1 to 3

The palm oil FAMEs 2, 3 and 5 shown in Table 1 were mixed into a commercially available diesel so as to be 20 mass % (bended diesel), and the increases in acid value, oxidation stabilities and pour points of the bended diesel were measured. The measurement results are shown in Table 5. From the results shown in Table 5, when palm oil FAME was used, there was no large difference in the increases in acid value, but when palm oil FAME having less polyunsaturated component was used, oxidative stability was improved and sludge formation and oil color change were not observed.

TABLE 5 pour point increase in acid precipi- oxidative of mixed value(mgKOH/g) tation oil color stability (min) diesel (° C.) Example 1 FAME-2 0.04 no pale yellow 107.8 −8 Example 2 FAME-3 0.02 no pale yellow 123.7 −8 Example 3 FAME-5 0.02 no pale yellow 145.6 −8 Comparative — 3.85 yes orange 70.3 −14 Example 1 Comparative FAME-1 3.98 yes orange 78.5 −8 Example 2

Comparative Examples 1 to 2

As Comparative Example 1, the same measurements as in Example 1 were carried out except that a diesel alone with no fatty acid methyl ester oil mixed was used. As Comparative Example 2, the same measurements as in Example 1 were carried out except that palm oil FAME-1 shown in Table 1 was used. The measurement results are shown in Table 5. From the results shown in Table 5, when a commercially available diesel alone was subjected to the forced oxidation test, the increase in acid value was as high as 3.85 mg KOH/g. On the other hand, by mixing 20% of FAME-1, the increase in acid value was slightly increased to 3.98 mg KOH/g. By forced oxidation, the sample oil was gradually oxidized and its color became deeper from pale yellow to yellow, and further to orange. When the sample oil after forced oxidation showed orange, formation of sludge was also confirmed. On the other hand, oxidative stability was 65 minutes or more in either case.

Examples 4 to 7 and Comparative Examples 3 to 5

As Examples 4 to 7, the palm oil FAME-7 shown in Table 1 was mixed into a commercially available diesel so as to be 1, 5, 10 or 20 mass %, and the increases in acid value and pour points of the mixed diesels were measured. As Comparative Examples 3 to 5, the same measurements as in Example 4 were carried out except that palm oil FAME-1 shown in Table 1 was mixed into a commercially available diesel so as to be 1, 5 or 10 mass %. The measurement results are shown in Table 6. From the results shown in Table 6, when FAME-1 with a large content of polyunsaturated component was mixed, the increase in acid value was enhanced with increase in the mixing ratio, and sludge formation was also observed at 10 mass % or more. On the other hand, when FAME-7 with a less content of polyunsaturated component was mixed, no increase in acid value was observed even when the mixing ratio of the FAME was increased, and also no sludge formation was observed.

TABLE 6 FAME pour point mixing increase in acid precipi- of mixed FAME ratio (%) value (mgKOH/g) tation oil color diesel (° C.) Example 4 FAME-7 1 0.01 No pale yellow −13 Example 5 FAME-7 5 0.03 No pale yellow −12 Example 6 FAME-7 10 0.02 No pale yellow −10 Example 7 FAME-7 20 0.02 No pale yellow −8 Comparative FAME-1 1 0.62 No yellow −13 Example 3 Comparative FAME-1 5 0.85 No yellow −12 Example 4 Comparative FAME-1 10 1.65 yes orange −11 Example 5

Examples 8 to 15 and Comparative Examples 6 to 16

As Examples 8 to 15, the palm oil FAME-7 shown in Table 1 was mixed into a commercially available diesel so as to be 1, 5, 10, 20, 30, 40, 50, or 70 mass %, and their oxidative stabilities and pour points were measured. In order to make the difference due to the difference in oil composition more obvious, the accelerated test was carried out with the forced oxidation temperature set at 125° C. As Comparative Examples 6 to 14, the same measurements as in Example 8 were carried out except that palm oil FAME-1 shown in Table 1 was mixed into a commercially available diesel so as to be 1, 5, 10, 20, 30, 40, 50, 70, or 80 mass %. As Comparative Example 15, the same measurements as in Example 8 were carried out except that palm oil FAME-7 shown in Table 1 was mixed into a commercially available diesel so as to be 80 mass %. As Comparative Example 16, the same measurements as in Example 8 were carried out except that a diesel alone with no fatty acid methyl ester oil mixed was used. The measurement results are shown in Table 7. From the results in Table 7, the forced-oxidation at 125° C. resulted in a significantly enhanced increase in acid value and an increased sludge formation even in a diesel alone. When FAME-1 with a large content of polyunsaturated component was used, the increase in acid value gradually increased with the increase in the mixing ratio to diesel, but the oxidative stability was slightly improved inversely. On the other hand, when FAME-7 with a less content of polyunsaturated component was used, the increase in acid value significantly decreased with the increase in the mixing ratio, and sludge formation was not observed at any mixing ratio. On the other hand, when FAME-7 was mixed into a commercially available diesel so as to be 80 mass % (Comparative Example 15), the increase in oxidation was very small, but the pour point of the mixed diesel showed a significantly increased value of 11° C. exceeding 10° C.

TABLE 7 FAME pour point mixing increase in acid precipi- oxidative of mixed FAME ratio (%) value (mgKOH/g) tation oil color stability (min) diesel (° C.) Example 8 FAME-7 1 6.32 no yellow 123.6 −13 Example 9 FAME-7 5 3.35 no pale yellow 156.4 −12 Example 10 FAME-7 10 1.31 no pale yellow 172.7 −10 Example 11 FAME-7 20 1.09 no pale yellow 186.2 −8 Example 12 FAME-7 30 0.76 no pale yellow 198.6 −5 Example 13 FAME-7 40 0.45 no pale yellow 201.3 −2 Example 14 FAME-7 50 0.12 no pale yellow 215.0 1 Example 15 FAME-7 70 0.08 no pale yellow 218.2 8 Comparative FAME-1 1 16.03 yes orange 71.3 −13 Example 6 Comparative FAME-1 5 18.04 yes orange 72.1 −12 Example 7 Comparative FAME-1 10 21.19 yes orange 76.7 −11 Example 8 Comparative FAME-1 20 34.37 yes orange 78.5 −8 Example 9 Comparative FAME-1 30 35.29 yes orange 78.7 −5 Example 10 Comparative FAME-1 40 37.80 yes orange 79.0 −3 Example 11 Comparative FAME-1 50 38.92 yes orange 79.5 −1 Example 12 Comparative FAME-1 70 40.06 yes orange 79.8 5 Example 13 Comparative FAME-1 80 41.23 yes orange 79.9 8 Example 14 Comparative FAME-7 80 0.06 no pale yellow 222.6 11 Example 15 Comparative 0 11.62 yes orange 70.3 −14 Example 16

Examples 16 to 21

The same measurements as in Example 8 were carried out except that palm oil FAMEs-2 to 6 and 8 shown in Table 1 were mixed into a commercially available diesel so as to be 20 mass %. The measurement results are shown in Table 8. From the results shown in Table 8, when FAMEs with a larger content of polyunsaturated component were mixed, inhibitory effects on the increase in acid value were observed, and also no sludge formation was observed. On the other hand, when FAME-8 with a large content of polyunsaturated component was used, the pour point of the mixed diesel increased to −7° C.

TABLE 8 pour increase point of in acid mixed value oil diesel FAME (mgKOH/g) precipitation color (° C.) Example FAME-2 28.08 yes orange −8 16 Example FAME-3 16.08 yes orange −8 17 Example FAME-4 14.20 yes orange −8 18 Example FAME-5 5.95 no yellow −8 19 Example FAME-6 2.31 no yellow −8 20 Example FAME-8 0.09 no pale −7 21 yellow

Examples 22 to 27 and Comparative Example 17

As Examples 22 to 27, the same measurements as in Example 1 were carried out except that rapeseed oil FAMEs-10 to 15 were mixed into a commercially available diesel so as to be 20 mass %. As Comparative Example 17, the same measurements as in Example 1 were carried out except that rapeseed oil FAME-9 shown in Table 2 was mixed into a commercially available diesel so as to be 20 mass %. The measurement results are shown in Table 9. From the results shown in Table 9, even when a rapeseed oil FAME was mixed, use of FAMEs with a less content of polyunsaturated component showed inhibitory effects on the increase in acid value and sludge formation and also resulted in increased oxidative stabilities. On the other hand, when FAMEs with higher depth of hydrogenation were used, the pour point of the mixed diesel increased.

TABLE 9 pour point increase in acid precipi- oxidative of mixed FAME value (mgKOH/g) tation oil color stability (min) diesel (° C.) Example 22 FAME-10 21.68 yes orange 44.9 −13 Example 23 FAME-11 10.80 no yellow 68.8 −12 Example 24 FAME-12 0.01 no pale yellow 100.2 −10 Example 35 FAME-13 0.05 no pale yellow 114.8 −8 Example 26 FAME-14 0.03 no pale yellow 117.9 −8 Example 27 FAME-15 0.01 no pale yellow 124.5 −6 Comparative FAME-9 24.93 yes orange 26.6 −13 Example 17

Examples 28 to 32 and Comparative Example 18

As Examples 28 to 32, the same measurements as in Example 1 were carried out except that soybean oil FAMEs-17 to 21 were mixed into a commercially available diesel so as to be 20 mass %. As Comparative Example 18, the same measurements as in Example 1 were carried out except that soybean oil FAME-16 shown in Table 3 was mixed into a commercially available diesel so as to be 20 mass %. The measurement results are shown in Table 10. From the results shown in Table 10, even when a soybean oil FAME was mixed, use of FAMEs with a less content of polyunsaturated component showed inhibitory effects on the increase in acid value and sludge formation and also resulted in increased oxidative stabilities. On the other hand, when FAMEs with higher depth of hydrogenation were used, the pour point of the mixed diesel increased.

TABLE 10 pour point increase in acid precipi- oxidative of mixed FAME value (mgKOH/g) tation oil color stability (min) diesel (° C.) Example 28 FAME-17 27.56 yes orange 40.2 −11 Example 29 FAME-18 1.49 no yellow 87.3 −10 Example 30 FAME-19 0.04 no pale yellow 121.7 −9 Example 31 FAME-20 0.00 no pale yellow 125.0 −8 Example 32 FAME-21 0.00 no pale yellow 132.2 −6 Comparative FAME-16 29.02 yes orange 17.2 −11 Example 18

Examples 33 to 35

As Examples 33 to 35, the same measurements as in Example 1 were carried out except that FAMEs-22 to 24 whose compositions were adjusted by adding methyl stearate and methyl oleate as reagents to palm oil FAME-1 were mixed into a commercially available diesel so as to be 20 mass %. The measurement results are shown in Table 11. From the results shown in Table 11, even when palm oil FAMEs whose compositions were adjusted by adding the reagents were mixed, use of FAMEs with a less content of saturated component showed inhibitory effects on the increase in acid value and sludge formation and also resulted in increased oxidative stabilities.

TABLE 11 pour point increase in acid precipi- oxidative of mixed FAME value (mgKOH/g) tation oil color stability (min) diesel (° C.) Example 33 FAME-22 0.08 no pale yellow 100.7 −8 Example 34 FAME-23 0.06 no pale yellow 121.6 −8 Example 35 FAME-24 0.05 no pale yellow 130.2 −8

Thus, the oxidation inhibitor for diesel comprising fatty acid methyl esters of the present invention is effective for improving the oxidative stability of diesel and suppressing sludge formation, and a diesel fuel comprising the oxidation inhibitor is a diesel fuel composition excellent in oxidative stability and low-temperature fluidity.

INDUSTRIAL AVAILABILITY

The oxidation inhibitor of the present invention can be used as an oxidation inhibitor for diesel. 

1. A method of inhibiting oxidation of diesel comprising adding to diesel an oxidation inhibitor for diesel which comprises a palm oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.5 mass %-7.0 mass % of polyunsaturated fatty acid methyl ester and 50 mass %-84 mass % of saturated fatty acid methyl ester.
 2. A method of inhibiting oxidation of diesel comprising adding to diesel an oxidation inhibitor for diesel which comprises a rapeseed oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.5 mass %-24.5 mass % of polyunsaturated fatty acid methyl ester and 7 mass %-50 mass % of saturated fatty acid methyl ester.
 3. A method of inhibiting oxidation of diesel comprising adding to diesel an oxidation inhibitor for diesel which comprises a soybean oil-derived fatty acid methyl ester, wherein said oxidation inhibitor contains 0.8 mass %-32 mass % of polyunsaturated fatty acid methyl ester and 15 mass %-56 mass % of saturated fatty acid methyl ester.
 4. A diesel fuel composition, which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of a palm oil-derived fatty acid methyl ester, a rapeseed oil-derived fatty acid methyl ester, and a soybean oil-derived fatty acid methyl ester, in an amount of 1.0 mass % or more and 70 mass % or less, wherein said palm oil-derived fatty acid methyl ester contains 0.5 mass %-7.0 mass % of polyunsaturated fatty acid methyl ester and 50 mass %-84 mass % of saturated fatty acid methyl ester, wherein said rapeseed oil-derived fatty acid methyl ester contains 0.5 mass %-24.5 mass % of polyunsaturated fatty acid methyl ester and 7 mass %-50 mass % of saturated fatty acid methyl ester, and wherein said soybean oil-derived fatty acid methyl ester contains 0.8 mass %-32 mass % of polyunsaturated fatty acid methyl ester and 15 mass %-56 mass % of saturated fatty acid methyl ester.
 5. The diesel fuel composition of claim 4, which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the palm oil-derived fatty acid methyl ester, the rapeseed oil-derived fatty acid methyl ester, and the soybean oil-derived fatty acid methyl ester, in an amount of 1.0 mass % or more and 20 mass % or less.
 6. The diesel fuel composition of claim 4, which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the palm oil-derived fatty acid methyl ester, the rapeseed oil-derived fatty acid methyl ester, and the soybean oil-derived fatty acid methyl ester, in an amount of more than 20 mass % and 50 mass % or less.
 7. The diesel fuel composition of claim 4, which comprises an oxidation inhibitor for diesel comprising at least one selected from the group consisting of the palm oil-derived fatty acid methyl ester, the rapeseed oil-derived fatty acid methyl ester, and the soybean oil-derived fatty acid methyl ester, in an amount of more than 50 mass % and 70 mass % or less.
 8. The diesel fuel composition of claim 4, which satisfies all the following conditions (a), (b), and (c): (a) the pour point is 9° C. or less; (b) the oxidative stability by PetroOxy method is 65 minutes or more; and, (c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.
 9. The diesel fuel composition of claim 5, which satisfies all the following conditions (a-1), (b), and (c): (a-1) a pour point is −8° C. or less; (b) an oxidative stability by PetroOxy method is 65 minutes or more; and, (c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.
 10. The diesel fuel composition of claim 6, which satisfies all the following conditions (a-2), (b), and (c): (a-2) a pour point is 4° C. or less; (b) an oxidative stability by PetroOxy method is 65 minutes or more; and, (c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours.
 11. The diesel fuel composition of claim 7, which satisfies all the following conditions (a), (b), and (c): (a) a pour point is 9° C. or less; (b) an oxidative stability by PetroOxy method is 65 minutes or more; and, (c) no sludge is formed after forced-oxidation test performed by supplying pure oxygen at 115° C. for 16 hours. 